doc: update markdown formatting for *.md files

Fix formatting that will change when run through upcoming remark preset.

PR-URL: https://github.com/nodejs/node/pull/40154


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>